It is known that turboprops, with one or more propellers, have better energy efficiencies than turboshaft engines.
However, most of the existing aircraft are propelled by turboshaft engines which, on the one hand, have a high fuel consumption and, on the other hand, are sources of environmental pollution via their emissions into the atmosphere.
Therefore, with respect to energy-saving and environmental pollution, it would be more advantageous to furnish aircraft with turboprops rather than with turboshaft engines.
However, in operation, turboprops are still noisier than turboshaft engines, so that the advantage in energy-saving and environmental pollution through emissions, from which benefit would be derived by replacing turboshaft engines with turboprops, would be accompanied by the disadvantage of increased noise pollution. The noise pollution thus achieved would be unacceptable and, in any case, difficult to make compatible with the regulations that are currently in force.
It is also known that, amongst the known turboprops, those comprising two contrarotating propellers (usually called “contrarotating prop fans”) are particularly attractive with respect to propulsive efficiency, on the one hand, and radial space requirement, on the other hand. Specifically, a turboprop with two contrarotating propellers has, for equal power, a propeller diameter that is at least 25% less than that of a single-propeller turboprop.
Such turboprops may advantageously be placed at the rear of the aircraft, so that the propellers are moved away from the cabin and the noise felt in the latter is reduced. It will be noted that such a rear installation of such a turboprop is favored, since the diameter of said propellers is reduced.
Furthermore, for example through GB-1 397 068, it is known how to use portions of the airframe of an aircraft in order to mask, downward, the noise generated by at least one turboshaft engine mounted at the rear of the aircraft, on the back of the latter.